1. Field of the Invention
The present invention relates to a fuel cell/gas turbine combined power generation system, and more particularly to such a combined power generation system having a high pressure phosphoric acid fuel cell and a gas turbine in combination. The present invention also relates to a method of operating such a fuel cell/gas turbine combined power generation system.
2. Description of the Prior Art
Generally, phosphoric acid fuel cells give higher power generation efficiencies at higher operation pressures. Hence, in fuel cell power generation plants with relatively large capacities, for example, 1,000 kW or higher, there have been widely adopted high pressure phosphoric acid fuel cells which operate with reactant gases pressurized to, e.g., about 4 to 8 Kg/cm.sup.2 G. Fuel cell power plants of this type include a high pressure reformer which reforms a feed gas such as natural gas by means of steam reforming to produce a hydrogen rich reformed fuel gas, which is sent to respective anodes of the fuel cells while air taken in from the open air is compressed by an air compressor to produce pressurized air, which is sent under pressure as an oxidizer gas to respective cathodes of the fuel cells so that power generation can be carried out.
There have been known various types of power generation systems. For example, a fuel cell power generation system is known which includes a gas turbine for driving an associated air compressor. The gas turbine, which is coaxially connected to the air compressor and drives it, recovers power or energy by use of, as a working fluid, exhaust combustion gas from a reformer and exhaust air from the fuel cell. The fuel in the reformer gets the heat necessary for a reforming reaction from combustion of the anode off gas and the cathode off air from the fuel cell during its operation, as described in, for example, Japanese Patent Application Publication No. 56231/1983. Japanese Patent Application Laying-open No. 168569/1990 discloses a fuel cell power generation system which includes an air compressor, a gas turbine and a generator or dynamo driven by the gas turbine to generate electric power, and uses the thus generated electric energy to energize an electric motor to drive the air compressor. Also, a fuel cell power generation system is known which includes an auxiliary burner provided in the middle of an exhaust combustion gas feed line from the reformer so that a shortage in power of the gas turbine can be supplemented by burning a fuel with air at the time of starting the operation of the fuel cell or while the fuel cell is being operated at a low load (cf., e.g., Japanese Utility Model Registration Application Laying-open No. 128175/1984, and Japanese Patent Application Publication No. 56231/1983).
Further, there is known, as a combined cycle for recovering waste heat from a fuel cell power generation plant, a fuel cell power generation system having the above-described gas turbine in which the gas turbine drives a generator to generate power while the exhaust gas from the gas turbine is used to drive a turbo compressor to get pressurized air to be fed to the fuel cell (Japanese Patent Application Laying-open No. 7772/1983). In addition, a fuel cell power generation system is known which includes a generator and an air compressor that are coaxially connected to and driven by a gas turbine, and which is provided with a separate waste heat recovery boiler that uses waste heat from the gas turbine as a heat source so that it can produce steam, thereby allowing the system to generate power by the use of a steam turbine as well as the generator (Japanese Utility Model Registration Application Laying-open No. 128175/1984).
In the aforementioned combined system which includes a power generation plant designed for a phosphoric acid fuel cell and in combination therewith a gas turbine which recovers waste heat (i. e., exhaust combustion gas from the reformer) in the form of power, the gas turbine operates at an inlet gas temperature of a maximum of about 300.degree. C. to about 400.degree. C., and generates relatively low power. Actually, almost all the output power of the gas turbine is consumed for driving the air compressor which feeds pressurized air to the air system of the fuel cell. As a result, even when the generator is driven with a surplus output by the gas turbine or the separately arranged steam turbine/generator combined system is driven using waste heat from the gas turbine as a heat source, the amount of generated power will be very small in terms of the total heat account within the whole system. Accordingly, use of the waste heat insufficient for power generation is obtained by the additional installment provided in the system to enable the system as a combined cycle.